1. Field of the Invention
The present invention relates to a thread profile measuring method of measuring a variety of characteristic values of a thread profile.
2. Description of Related Art
A thread profile of a threaded object (female thread=screw hole, male thread=screw shaft) is typically specified based on a variety of characteristic values (e.g., a thread pitch and a pitch diameter), which are defined for each type of threaded object in accordance with JIS (Japan Industrial Standard).
For instance, JISB0205 defines a variety of characteristic values of a general metric thread. The characteristic values of a thread profile are herein defined in relation to a thread contour seen in a cross section taken along a plane including the center axis of the thread profile.
Typically, a three-wire method has been frequently used for measurement of a thread profile. According to the three-wire method, which is defined in accordance with JISB0261 or the like, one measurement needle hung on one side of a screw shaft and two measurement needles hung on the other side are each set between thread crests and an outer dimension around these needles is measured with a micrometer or the like to calculate a pitch diameter of the thread profile or the like.
However, the three-wire method requires a complicated process for preparing the measurement needles, positioning the measurement needles on the thread profile, and performing measurement and/or calculation.
In view of the above, there is suggested a method of measuring a variety of characteristic values of a thread profile using a coordinate measuring machine (see Patent Literature 1: JP-A-2001-82952).
Patent Literature 1 describes that a scanning probe provided to the coordinate measuring machine is moved along the center axis of a thread profile for scanning measurement of a thread contour to obtain a variety of characteristic values of the thread profile. In particular, Patent Literature 1 describes that the scanning measurement using the scanning probe is performed at a plurality of points defined in a circumferential direction of the thread profile to obtain three-dimensional profile information on the thread profile, based on which the pitch diameter of the thread profile can be calculated and the three-axial position and inclination of the center axis of the thread profile can be measured.
However, since the measurement using the coordinate measuring machine has to be repeated at the plurality of points to measure the thread profile, the method as described in Patent Literature 1 necessarily requires complicated device and process.
In view of the above, in order to simplify a device and a process for measurement, a uniaxially movable profile measuring instrument is also used to measure a thread profile (see Patent Literature 2: JP-A-06-185952).
Patent Literature 2 describes that a scanning measurement axis of the profile measuring instrument is aligned with the center axis of the thread profile and the thread is traced with a stylus of the profile measuring instrument in such a manner that thread crests are sequentially overpassed to obtain the thread contour, the thread pitch and the like.
By performing the scanning measurement one time using the uniaxially movable profile measuring instrument as described in Patent Literature 2, it is only possible to obtain limited characteristic values, such as the thread contour and the thread pitch that are defined along the center axis of the thread profile, but the pitch diameter and the like seen in the cross section intersecting with the center axis cannot be obtained.
In view of the above, in order to also measure the pitch diameter of the thread profile, the scanning measurement may be performed using the uniaxially movable profile measuring instrument at each of opposite positions opposed to each other across the center axis of the thread profile.
However, when the scanning measurement is performed at each of the opposite positions in the thread profile, it is necessary to perform adjustments such as so-called “alignment” and “vertex detection” for accurately aligning the center axis of the thread profile with the scanning axis of the profile measuring instrument (moving axis or trace locus of the scanning measurement).
The alignment, which is also referred to as leveling, is an operation for adjusting the scanning axis and the center axis of the thread profile to be parallel to each other.
When the alignment fails to be accurately performed, in other words, when the moving axis of the scanning measurement is not parallel to the center axis of the thread profile with an inclination between these two axes, the measured thread contour is deformed and thus the characteristic values of the thread profile cannot be accurately obtained.
Incidentally, Patent Literature 2 also describes that an inclination between these axes results in an error in the thread contour and thus the orientation and position of the profile measuring instrument are adjusted to align these axes with each other. However, Patent Literature 2 fails to describe a specific process for such an adjustment or the like.
When, for instance, a thread profile of a male thread (i.e., a screw shaft), which inherently includes a cylindrical portion, is to be measured, a direction of the center axis can be obtained by detecting a generatrix of a surface of the cylindrical portion for the alignment.
However, when no cylindrical portion is formed or when the thread profile of a female thread (i.e., a screw hole) is to be measured, it is difficult to perform the alignment based on a detected generatrix and thus it is requisite to locate the center axis by a cut-and-try method, which makes the process significantly complicated.
The vertex detection, which is also referred to as a peak/bottom detection, is an operation for detecting a peak (an upper vertex) and a bottom (a lower vertex) of a thread profile in a cross section (a plane intersecting with the center axis) to locate opposite positions (two positions used to detect the diameter) opposed to each other across the center axis.
When the vertex detection fails to be accurately performed, irrespective of whether or not the above-described alignment is accurately performed, it is not possible to obtain an accurate thread contour and to accurately calculate the pitch diameter and the like.
For instance, for measuring a thread profile of a male thread (i.e., a screw shaft), the scanning measurement is performed at each of the uppermost position (peak) and the lowermost position (bottom) of the thread profile in a cross section (a plane intersecting with the center axis). A thread contour obtained at each position, which is considered to be identical with a contour seen in the cross section including the center axis of the thread profile, is congruent with a thread profile defined in accordance with JISB0205 or the like.
However, when the scanning measurement is performed at positions deviated from the mutually opposite positions in the cross section of the threaded object (i.e., in the plane intersecting with of the center axis) because of an inaccurate vertex detection, an obtained thread contour is different from an actual thread.
As described above, when the scanning measurement is performed at each of the opposite positions opposed to each other across the center axis of the thread profile using the uniaxially movable profile measuring instrument, complicated device and process as described in Patent Literature 1 can be simplified. However, such scanning measurement performed at the opposite positions in the thread profile disadvantageously requires a complicated process for the “alignment” and the “vertex detection”.